Power conversion system

ABSTRACT

A power conversion system as an aspect of the present invention includes: a power converter using a plurality of legs to convert inputted electric power and output powers in a plurality of phases, each leg including upper and lower arms; and a controller  30  controlling the upper and lower arms of each leg to control pulse current flowing through the leg. The controller  30  calculates a duty ratio instruction of each leg in one control period for each phase and, concerning first and second legs among the plurality of legs provided for a certain one of the phases, changes the phase of the calculated duty ratio instruction so that a time period when positive pulse current flows through the first leg and a time period when negative pulse flows through the second leg overlap each other in the one control period.

TECHNICAL FIELD

The present invention relates to a power conversion system whichconverts inputted electric power to outputs in plural different phases.

BACKGROUND ART

As one of conventional power conversion systems, a motor control systemwhich supplies output power to a polyphase motor is known. PatentLiterature 1 discloses a power conversion system including sixthree-phase inverters. In this power conversion system, a pulsegenerator compares instruction values with referential values whichperiodically change and then supplies driving signals corresponding torespective phases to each of the six inverters. In this case, the phasesof the referential values for the respective inverters periodicallychanging are offset from each other. This can reduce vibrating currentof a DC common wiring section.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Unexamined Publication No.    2008-099436

SUMMARY OF INVENTION

However, in the case where power of a same phase is converted by pluralinverters like the method disclosed by PTL 1, the periods when the upperor lower arm in the legs of the same phase is turned on overlap eachother in some cases. The currents flowing through the legs in thepositive or negative direction are superimposed on each other, thuscausing a disadvantage of an increase in ripple current.

The present invention was made in the light of the aforementionedcircumstances, and an object of the present invention is to reduceripple current in a polyphase power converter including plural legs foreach phase.

In order to solve the aforementioned problems, according to the presentinvention, concerning first and second legs among a plurality of legsincluded in a certain phase constituting a power converter, the timeperiod when positive pulse current flows through the first leg and thetime period when negative pulse current flows through the second legoverlap each other in one control period.

According to the present invention, by constituting a same phase of aplurality of legs, the operating current of each leg can be reduced, andripple current can be reduced. Moreover, in one control period of a samephase, positive pulse current flowing through a certain leg and negativepulse current flowing through another leg overlap each other in timing.Accordingly, it is prevented that currents in the same direction aresuperimposed on each other. It is therefore possible to reduce ripplecurrent.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view schematically showing the entireconfiguration of a motor control system according to a first embodiment.

FIGS. 2( a) and 2(b) are explanatory views showing a configuration of anelectro-mechanical motor, FIG. 2( a) illustrating a detailedconfiguration of a motor 10, FIG. 2( b) illustrating a detailed circuitconfiguration of an inverter 20.

FIG. 3 is a block diagram schematically showing a configuration of acontroller 30.

FIG. 4 is an explanatory view showing transition of each phase current.

FIG. 5 is an explanatory view showing transition of pulse currentsflowing though each phase and each leg in one control period at a time Ashown in FIG. 4.

FIG. 6 is an explanatory view showing transition of pulse currentsconcerning a control mode in which carrier phase is shifted as anexample comparative to the control mode according to the firstembodiment.

FIGS. 7( a) to 7(c) are explanatory views showing superimposition ofcurrents of the legs when the duty ratio is 50%, FIG. 7( a) showing acase of an inverter including a leg for each of three phases, FIG. 7( b)showing a case where an inverter includes four legs for each of threephases and the phase of a duty ratio instruction for each leg is notchanged, and FIG. 7( c) showing a case where an inverter includes fourlegs for each of three phases and the phase of the duty ratioinstruction of each leg is changed.

FIG. 8 is an explanatory view showing a configuration of an inverter 20according to a second embodiment.

FIG. 9 is an explanatory view showing transition of pulse currentsflowing through each phase and each leg in one control period at acertain time.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 1 is an explanatory view schematically showing an entireconfiguration of a motor control system according to a first embodiment.The motor control system according to the first embodiment is a motorcontrol system controlling a driving motor of an electric vehicle. Thismotor control system mainly includes a motor 10, an inverter 20 as apower converter, and a controller 30.

The motor 10 mainly includes a rotor and a stator. The motor 10 is apermanent magnet synchronous motor including n-phase (n: a naturalnumber not less than 1) phase windings connected in a star configurationaround a neutral point, the phase windings being wound around teeth ofthe stator (in this embodiment, a three-phase motor having phases U, V,and W (n=3)). In this embodiment, each phase winding is divided into mparts according to the number of slots of the motor 10. The windingsconcerning a same phase are properly wound around predetermined statorcores. Hereinafter, m sets of elements of the phase U (the winding andlater-described legs) are indicated as phases U1, U2, . . . , Um, and msets of elements of the phases V and W are indicated as phases V1, V2,Vm and phases W1, W2, . . . , Wm, respectively.

The motor 10 is driven by an interaction between a magnetic fieldgenerated by three-phase alternating current power supplied from alater-described inverter 20 to the respective phase windings and amagnetic field generated by permanent magnets of the rotor. The rotorand output shaft joined thereto are therefore rotated. The output shaftof the motor 10 is joined to an automatic transmission of an electricvehicle, for example.

The inverter 20 is connected to a power supply 5. The inverter 20converts DC power received from the power supply 5 to AC powers andsupplies the same to the motor 10. The AC powers are generated by eachphase. The AC powers of respective phases generated by the inverter 20are individually supplied to the motor 10. The input side of theinverter 20 is connected to the power supply 5 through a smoothingcapacitor C.

The inverter 20 includes m legs connected in parallel for each of thephases U, V, and W. Specifically, the phase U includes m legscorresponding to the phases U1 to Um, the legs being connected inparallel. In a similar manner, the phase V (phase W) includes m legscorresponding to the phases V1 (W1) to Vm (Wm), the legs being connectedin parallel. Each of the legs of each phase includes an upper armconnected to a bus on the positive side of the power supply 5 and alower arm connected to the negative side of the power supply 5, theupper and lower arms being connected in series. Each of the armsconstituting each leg mainly includes a semiconductor switch capable ofcontrolling one-way conduction (a switching element such as a transistorincluding IGBT, for example). The semiconductor switch is connected to afreewheeling diode in inverse parallel.

The on/off state of each arm, or the on/off state of each semiconductorswitch (switching operation) is controlled through a driving signaloutputted from the controller 30. The semiconductor switch constitutingeach arm is turned on by the driving signal from the controller 30 intoconduction and turned off into non-conduction (the cutoff state).

In the first embodiment, the motor 10 and the power converter (inverter20) are integrated and implemented as an electro-mechanical motor. Forexample, as shown in FIG. 2( a), in the three-phase motor 10, a stator12 located around the outer periphery of the rotor 11 includes six slotsS1 to S6. By taking out two wirings from each of the slots S1 to S6, themotor 10 is connected to four legs of each phase, that is, 12 legs (theintegral multiple of the number of slots) in all.

Referring back to FIG. 1, the controller 30 controls the switchingoperation of the inverter 20 to control the output torque of the motor10. The controller 30 can be composed of a microcomputer mainlyincluding a CPU, a ROM, a RAM, and an I/O interface. The controller 30performs operations to control the inverter 20 according to a controlprogram stored in the ROM. The controller 30 outputs control signals(driving signals) calculated by the above operations to the inverter 20.

The controller 30 receives sensor signals detected by various sensors. Aposition sensor (a resolver, for example) 40 is attached to the motor 10and detects an electric phase (electric angle) θ through positionalinformation indicating the rotor position of the motor 10. Moreover, acurrent sensor 41 is a sensor detecting actual current flowing througheach phase. Specifically, the current sensor 41 detects actual currentsflowing through m phase-windings of each phase (hereinafter,collectively referred to as actual currents Inm).

The controller 30 controls the switching operation of the inverter 20,that is, controls on/off states of the upper and lower arms constitutingeach leg on a phase-by-phase basis by a control method such as PWM wavevoltage drive. The PWM wave voltage drive is a method of generating PWMwave voltage from DC voltage by PWM control and applying the generatedPWM wave voltage to the motor 10. Specifically, the PWM wave voltagedrive is a driving method of calculating a duty ratio instruction valuebased on the carrier signal and voltage instruction value of each phaseat each control period to apply an equivalent sinusoidal AC voltage tothe motor 10.

FIG. 3 is a block diagram schematically showing a configuration of thecontroller 30. From a functional perspective, the controller 30 includesa torque control unit 31, a current control unit 32, a dq/three-phaseconversion unit 33, a modulation factor instruction generation unit 34,a PWM control unit 35, a timing control unit 36, a three-phase/dqconversion unit 37, and a rotation speed calculation unit 38.

Based on a torque instruction T given from the outside (from acontroller of a vehicle, for example) and a motor rotation speed ω, thetorque control unit 31 calculates a d-axis current instruction and aq-axis current instruction corresponding to the given torque instructionT (collectively referred to as a dq-axis current instruction idq). Thetorque control unit 31 holds a map defining the relations between atorque instruction value T or motor rotation speed co and the dq-axiscurrent instruction idq. These relations are previously acquired byexperiments and simulations in consideration of the characteristics ofthe motor 10 and the like. The torque control unit 31 refers to the mapand calculates the dq-axis current instruction idq. The calculateddq-axis current instruction idq is outputted to the current control unit32. Herein, the motor rotation speed ω which is necessary to calculatethe dq-axis current instruction idq can be obtained as the result ofcalculation by the rotation speed calculation unit 37. This rotationspeed calculation unit 37 calculates the electric angular speed, thatis, the motor rotation speed ω by differentiating the electric angle θ,which is detected by the position sensor 40, with respect to time.

The current control unit 32 first calculates the d-axis and q-axiscurrent deviations. To be specific, in addition to the dq-axisinstruction idq, the current control unit 32 receives the d-axis actualcurrent and q-axis actual current corresponding to the three-phaseactual current Inm (collectively referred to as dq-axis actual currentsIdq). Herein, the dq axis actual currents Idq are calculated in such amanner that the three-phase/dq conversion unit 38 performs coordinateconversion for the three-phase actual currents Inm based on the electricangle θ detected by the position sensor 40. The current control unit 32calculates the d-axis and q-axis current deviations by subtracting thedq-axis actual currents Idq from the dq-axis current instructions idqfor the d-axis and q-axis. The current control unit 32 uses PI control,for example, to calculate the d-axis and q-axis voltage instructions(collectively referred to as dq-axis voltage instructions Vdq) so thatthe d-axis and q-axis current deviations are 0. The calculated dq-axisvoltage instructions Vdq are outputted to the dq/three-phase conversionunit 33.

The dq/three-phase conversion unit 33 refers to the electric angle θdetected by the position sensor 40 to perform coordinate conversion fromthe dq-axis voltage instructions vdq to voltage instructionscorresponding to the three phases, or a U-phase voltage instruction, aV-phase voltage instruction, and a W-phase voltage instruction(collectively referred to as three-phase voltage instructions vn). Thethree-phase voltage instructions vn are individually outputted to themodulation factor generation unit 34.

The modulation factor generation unit 34 standardizes the three-phasevoltage instructions vn with the power supply voltage to calculatemodulation factor instructions of the respective phases, a U-phasemodulation factor instruction, a V-phase modulation factor instruction,and a W-phase modulation factor instruction (collectively referred to asthree-phase modulation factor instructions Mn). The calculatedthree-phase modulation factor instructions Mn are outputted to the PWMcontrol unit 35.

The PWM control unit 35 compares the signal level of the carrier signalperiodically varying, such as a triangular wave, and the three-phasemodulation factor instruction Mn at each control period. Based on thecomparison results, the PWM control unit 35 generates driving signals toturn on/off the semiconductor switches of the inverter 20. To bespecific, if the signal level of a carrier signal is lower than thethree-phase modulation factor instruction Mn, the PWM control unit 35outputs a driving signal to turn on the corresponding upper arm and adriving signal to turn off the corresponding lower arm. On the otherhand, if the signal level of a carrier signal is higher than thethree-phase modulation factor instruction Mn, the PWM control unit 35outputs a driving signal to turn off the corresponding upper arm and adriving signal to turn on the corresponding lower arm. In other words,each driving signal corresponds to a duty ratio instruction for a leg(the semiconductor switches of the upper and lower arms) in one controlperiod and is generated for each phase. In the first embodiment, sinceeach phase is composed of m legs, the driving signal of each phase isdivided into m signals, and driving signals Sp_nm corresponding to the mlegs are generated for each phase. The generated driving signals Sp_nmare outputted to the timing control unit 36. In order to prevent thatthe semiconductor switches of the upper and lower arms aresimultaneously turned on, the PWM control unit 35 can set a dead time,that is, a time period when both of the semiconductor switches of a legare off, between the end of on operation of the semiconductor switch ofone of the upper and lower arms (off timing) and the start of onoperation of the semiconductor switch of the other arm (on timing).

The timing control unit 36 changes the phase of the driving signal Sp_nmat least one of the m legs of a same phase in one or some of the phasesU, V, and W. As one of the characteristics of the first embodiment,concerning first and second legs among the m legs provided for a certainphase, the timing control unit 36 changes the phase of the duty ratioinstruction of the first or second leg so that in one control period, atime period when positive pulse current flows through the first leg anda time period when negative pulse current flows through the second legoverlap each other. In other words, the positive pulse current flowingthrough the first leg and the negative pulse current flowing through thesecond leg overlap each other in timing. Hereinafter, a description isgiven of the phase change of the timing control unit 36 in detail.

FIG. 4 is an explanatory view showing transition of each phase current.FIG. 5 is an explanatory view showing transition of pulse currentflowing through each phase and each leg during one control period attiming A. Herein, the current values shown in FIG. 5 are 1/10 of actualvalues for the sake of convenience. The inverter 20 includes five legsfor each phase and allows current of up to 100 A through each leg. Inother words, in the inverter 20, the phase current for each phase is thesum of currents of five legs, that is, 500 A. In this specification,among the currents flowing through legs, the positive pulse currentrefers to actual current flowing through a leg in such a direction thatthe capacitor C is discharged. The negative pulse current refers toactual current flowing through a leg in such a direction that thecapacitor C is charged. Moreover, the boxed numerals among the values ofcurrents flowing through the individual legs shown in FIG. 5 representthe values of currents when the lower arms are on, and the unboxednumerals represent the values of currents when the upper arms are on.The same goes with FIGS. 6 and 9 described later.

(First Condition)

It is controlled so that, concerning arbitrary two legs constituting asame phase, for example, the legs of the phases U1 and U2, the timeperiod when negative pulse current is flowing through the leg of thephase U1 and the time period when the positive pulse current is flowingthrough the leg of the phase U2 overlap each other. The aforementionedrelation is satisfied not only between the phases U1 and U2 but alsobetween the phases U2 and U3, between the phases U3 and U4, and betweenthe phases U4 and U5. Furthermore, the above relation is satisfied notonly in the phase U but also in the phase V.

(Second Condition)

It is controlled so that the duty ratios (duty ratio instructions) ofarbitrary two legs constituting a same phase, for example, the legs ofthe phases U1 and U2, are equal to each other. The aforementionedrelation is satisfied not only in the relation between the phases U1 andU2 but also in the relations between the phases U2 and U3, between thephases U3 and U4, and between the phases U4 and U5. Furthermore, theabove relation is satisfied not only in the phase U but also in thephase V.

(Third Condition)

In a certain phase, for example, in the phase U, the width (period) ofpositive pulse current of the phase U1 is long and the width (period) ofthe negative pulse current is short. In this case, it is controlled sothat the short period of the negative pulse current of the phase U2falls within the long period of the positive pulse current of the phaseU1. The above-described relation is satisfied not only between thephases U1 and U2 but also between the phases U2 and U3, between thephases U3 and U4, and between the phases U4 and U5. Furthermore, theabove relation is satisfied not only in the phase U but also in thephase V.

(Fourth Condition)

It is configured so that the U-phase current, or, the time period whenthe sum of the currents of the phases U1 to U5 is minimized and theV-phase current, or the time period when the sum of the currents of thephases V1 to V5 is minimized overlap each other. In other words, it iscontrolled so that the time period when the total ripple current of acertain phase is minimized does not overlap the time period when thetotal ripple current of another phase is minimized.

(Fifth Condition)

At time A1, the lower arms of the legs of the phases U3, V2, and W1 toW5 are turned on among the legs of the three phases. On the other hand,at time A2, the lower arms of the legs of the phases U4, V2, and W1 toW5 are turned on among the legs of the three phases. It is controlled sothat among all the legs provided for the inverter 20, the number of legswhose upper or lower arms are on remains constant throughout one controlperiod. Moreover, in order to keep constant the number of legs whoselower arm is on during one control period in all the three phases, it iscontrolled so that the number of arms through which negative pulsecurrent flows or the number of arms through which positive pulse currentflows is substantially constant throughout one control period.

(Sixth Condition)

As shown in a period surrounded by a solid line ellipse, the time whenthe on-state of the lower arm changes to the on-state of the upper armin the phase U1 corresponds to the time when the on-state of the upperarm changes to the on-state of the lower arm in the phase U2. As shownin a period surrounded by a dashed-line ellipse, the time when theon-state of the lower arm changes to the on-state of the upper arm inthe phase U2 corresponds to the time when the on-state of the upper armchanges to the on-state of the lower arm in the phase U3. As apparentfrom this form, it is controlled so that in a same phase, the periodwhen the positive pulse current flows through one leg (the leg of thephase U1, for example) ends at the same time as the period when thepositive pulse current flows through another leg (the leg of the phaseU2, for example) starts. In other words, it is controlled so that thepositive pulse current flowing through one leg is continuous with thepositive pulse current flowing through another leg. The above-describedrelation is satisfied not only in the relation between the phases U1 andU2 but also in the relations between the phases U2 and U3, between thephases U3 and U4, and between the phases U4 and U5. Furthermore, theabove relation is satisfied not only in the phase U but also in the legsof the phases V1 to V5 of the phase V.

Based on the aforementioned viewpoints, the timing control unit 36changes the phase of the driving signal Sp_nm of at least one of m legsof a same phase in any one or some of the phases U, V, and W. The timingcontrol unit 36 outputs driving signals Spa_nm to the inverter 20 at thetime of changing the phase. In the inverter 20, the m legs of each phasetherefore perform switching operations according to the driving signalsSpa_nm, so that predetermined pulse currents flows through the legs at apredetermined time. Accordingly, predetermined voltage is applied to themotor 10, thus driving the motor 10.

As described above, according to the first embodiment, the inverter 20is configured so that, concerning a first leg (the leg of the phase U1,for example) and a second leg (the leg of the phase U2, for example)among a plurality of legs provided for a certain phase (the phase U, forexample), the positive pulse current flowing through the first leg andthe negative pulse current flowing through the second leg in one controlperiod overlap each other in timing. In other words, concerning thefirst and second legs, the controller 30 changes the phases of the dutyratio instructions Sp_nm of the first and second legs so that thepositive pulse current flowing through the first leg and the negativepulse current flowing through the second leg overlap each other intiming during one control period.

Herein, FIG. 6 shows a control mode to shift the carrier phase as anexample comparative to the control mode according to the firstembodiment. Similar to FIG. 5, FIG. 6 shows transition of pulse currentsflowing through each leg and each phase during one control periodcorresponding to the timing A shown in FIG. 4. Herein, the control toshift the carrier phase, shown in the comparative example, is a controlmode in which the duty ratio instructions concerning respective legsincluded in a same phase are calculated using carriers whose phases areoffset from leg to leg.

In a period surrounded by a solid-line ellipse concerning the phases U1and U2, the lower arms of the phases U1 and U2 are both on. In thisperiod, the negative pulse current of the phase U1 and the positivepulse current of the phase U2 do not overlap each other in timing.Accordingly, the currents in the negative direction of the differentlegs are superimposed on each other, and the current in the negativedirection tends to increase. Moreover, in the period surrounded by asolid-line ellipse concerning the phases U2 and U3 or phases V2 and V3,the time periods when the lower arms of two legs included in a samephase are on overlap each other, in addition, the time periods when thelower arms of two legs included in different phases are on overlap eachother. Accordingly, the current in the negative direction tends to befurther increased. In this case, the total ripple current of all thelegs has an effective value of about 92 Arms, which is large.

In this term, according to the first embodiment, by performing a controlas previously described, it is prevented that currents in the negativedirection or currents in the positive direction are superimposed on eachother. This can reduce the ripple current. In other words, byconstituting a same phase of a plurality of legs, it is possible toreduce the operating current of each leg and therefore reduce the ripplecurrent. Moreover, if focusing one control period of a same phase, theperiod when positive pulse current flows through a certain leg and theperiod when negative pulse current flows through another leg overlapeach other. Accordingly, it is possible to prevent that currents in asame direction are superimposed on each other, thus reducing ripplecurrent.

Moreover, in this embodiment, the duty ratio instructions for the firstand second legs are set equal to each other. This configuration canprevent that the values of currents flowing through the first and secondlegs are different from each other in one control period. Accordingly,it is possible to prevent degradation of the performance in torquecontrol of the motor 10.

When the duty ratio is 50% (the on and off periods are equal), as shownin FIGS. 7( a) to 7(c), as for the phases U1 and U2, the positive pulsecurrent of the phase U1 and the negative pulse current in the phase U2completely overlap each other (the same applies to the phases U3 andU4). Among FIGS. 7( a) to 7(c), in FIG. 7( a), it is assumed that in aninverter, each of the three phases is composed of one leg, showingcurrent flowing through a leg of the phase U (the upper view), totalcurrent of the phase U (the middle view), and current flowing throughthe capacitor C. On the other hand, in FIGS. 7( b) and 7(c), it isassumed that in each inverter, each of the three phases is composed offour legs, schematically showing current flowing through each leg of thephase U (the upper view), the total current of the phase U (the middleview), and current flowing through the capacitor C. Herein, FIG. 7( b)shows a state where the phases of the duty ratio instructions for eachleg are not changed, and FIG. 7( c) shows a state where the phases ofthe duty ratio instructions of some legs are changed as described in thefirst embodiment. In the drawing, each hatched region indicated bydiagonal lines sloping up to the right shows a state where the upper armis on, and the hatched region indicated by lines sloping down to theright shows a state where the lower arm is on.

Furthermore, in the first embodiment, the controller 30 compares theperiod of positive pulse current flowing through the first leg with theperiod of the negative pulse current flowing through the second leg andthen changes the phases of the duty ratio instructions so that the longone of the compared periods of pulse current falls within the short one.With such a configuration, at any duty ratio varied at each controlperiod, superimposition of currents in the negative direction orcurrents in the positive direction can be prevented, thus reducingripple current.

Still furthermore, in the first embodiment, the controller 30 changesthe phases of the duty ratio instructions so that the period when thetotal ripple current of a certain phase is minimized and the period whenthe total ripple current of another phase is minimized do not overlapeach other. With such a configuration, it is possible to preventsuperimposition of currents in the negative direction or positivedirection in the legs constituting the respective phases, thus reducingthe ripple current.

Furthermore, in the first embodiment, the controller 30 changes thephases of the duty ratio instructions so that the positive pulse currentflowing through the first leg is continuous with the positive pulsecurrent flowing through the second leg. With such a configuration, it ispossible to prevent ripple current caused between the positive pulsecurrent in the first leg and the positive pulse current in the secondleg.

With such a configuration, it is controlled so that the number of armsdriven to transmit the positive pulse current or the number of armsdriven to transmit negative pulse current remains substantially constantalong time transition during one control period. This reduces thesuperimposition of the currents in the positive or negative direction inthe legs constituting each phase. Accordingly, the sum of ripplecurrents can be reduced. For example, in the example shown in FIG. 6, attime B1, the lower arms of the legs of the phases U4, V4, and W1 to W5are on among the legs of the three phases. On the other hand, at timeB2, the lower arms of the legs of the phases U4, U5, V4, V5, and W1 toW5 are on among the legs of the three phases. The total number of lowerarms which are on in the three phases changes in such a manner. However,according to the first embodiment, the aforementioned situation can beprevented.

Still furthermore, in the first embodiment, the motor 10 is anelectro-mechanical motor integrally including the motor 10 and inverter20. This electro-mechanical motor includes a plurality of windings and aplurality of bridge circuits composed of a plurality of legs. The outputpoint of each leg is connected to the corresponding winding. Forexample, in the configuration which is divided into separate mechanicaland electrical units, even if the legs provided for a same phase areconnected in parallel, the phases of the pulse currents of the legsconcerning a same phase cannot be offset from each other. On thecontrary, if the pulse currents are not synchronized, the current couldbe concentrated to one of the legs. In this case, it is necessary tosynchronize the timings in accordance with the operation characteristicsof the semiconductor switches provided with arms, thus complicating thecontrol. In this regard, according to the first embodiment, the motor 10can be composed as an electro-mechanical motor. The plurality ofwindings divided in each phase can be therefore connected to theplurality of bridge circuits of the inverter 20. Accordingly, by using apolyphase power converter including a number of legs connected inparallel for each phase, the aforementioned control can be effectivelyimplemented.

In the first embodiment, the inverter 20 includes legs the number ofwhich is an integral multiple of the number of motor slots. With thisconfiguration, the number of legs of a same phase can be increased, andthe current flowing through each leg can be reduced. It is thereforepossible to effectively prevent occurrence of ripple current.

Second Embodiment

Hereinafter, a description is given of a motor control system accordingto a second embodiment of the present invention. The motor controlsystem according to the second embodiment differs from that of the firstembodiment in including phase windings of five phases and an inverter 20including m legs for each phase, the m legs being connected in parallel.The description of the same matters as those of the first embodiment isomitted, and hereinafter, the description focuses on the differences.

The motor 10 is a permanent magnet synchronous motor including n (n: anatural number not less than 1) phase windings which are wound aroundteeth of the stator (in this embodiment, a five-phase motor havingphases U, V, W, X, and Y). Each phase winding is divided into m parts.The windings concerning a same phase are properly wound aroundpredetermined stator cores. Hereinafter, m sets of elements of the phaseU (the winding and later-described leg) are indicated as phases U1, U2,. . . , Um, and elements of phases V to Y are also indicated as phasesV1 to Y1, V2 to Y2, . . . , and Vm to Ym, respectively. In the secondembodiment, the description is given for m=5.

As shown in FIG. 8, the inverter 20 includes five legs for each of thephases U, V, W, X, and Y, the legs being connected in parallel. To bespecific, the phase U is provided with five legs corresponding to thephases U1 to U5. Similarly, the phases V, W, X, and Y are each providedwith five legs corresponding to the phases V1, W1, X1, and Y1 to V5, W5,X5, and Y5, respectively. The five legs of each phase are connected inparallel. Each of the legs constituting each phase includes an upper armconnected to a bus on the positive electrode side of the power supply 5and a lower arm connected to a bus on the negative electrode side of thepower supply 5, the upper and lower arms being connected in series. Eachof the arms constituting each leg is mainly composed of a semiconductorswitch capable of controlling one-way conduction (a switching elementsuch as a transistor including IGBT, for example). The semiconductorswitch is connected to a free-wheeling diode in inversed parallel.

Even in an electro-mechanical motor having the aforementionedconfiguration, similarly to the first embodiment, the timing controlunit 36 of the controller 30 changes the phase of the driving signalSp_nm of at least one of the five legs of a same phase in any one orsome of the phases U to Y.

FIG. 9 is an explanatory view showing transition of pulse currentsflowing through each leg and each phase during one control period at acertain time. In FIG. 9, the instantaneous current of the phase U is 50A; the phase V, 98 A; the phase W, 10 A; the phase X, −91 A; and thephase Y, 67 A. The values of current shown in FIG. 9 are 1/10 of actualvalues.

In the phase V with large current, it is controlled so that negativepulse current flowing through the leg of the phase V1 and positive pulsecurrent flowing through the leg of the phase V2 overlap each other intiming. The above-described relation is satisfied not only between thephases V1 and V2 but also between the phases V2 and V3, between thephases V3 and V4, and between the phases V4 and V5. Furthermore, thesame relation is satisfied not only in the phase V but also in thephases X and Y, which involve large current.

In this case, it is controlled so that the time period when the totalripple current of the phase V is minimized (the period surrounded by asolid-line ellipse in the drawing) does not overlap the time period whenthe total ripple current of the phase X is minimized (the periodsurrounded by another solid-line ellipse in the drawing).

Moreover, in order to reduce the total ripple currents of the phases Uto Y, it is controlled so that the time period of negative pulse currentof the phase U, which is a phase involving the second smallest current(the period surrounded by a dashed-line ellipse in the drawing), doesnot overlap the period of negative pulse current of the phase W, whichis a phase involving the smallest current.

According to the second embodiment, in one control period, the timeperiod when positive pulse current flows through a leg (a first leg) ofa certain phase and the time period when negative pulse current flowsthrough another leg (a second leg) of the same phase overlap each other.With such a configuration, similarly to the first embodiment, thesuperimposition of currents in the negative or positive direction isprevented, thus reducing the ripple current.

Moreover, in the second embodiment, the controller 30 changes the phasesof the duty ratio instructions so that positive pulse current flowingthrough the first leg is continuous with positive pulse current flowingthrough the second leg. With such a configuration, it is possible toprevent ripple current which could be generated between the positivepulse current of the first leg and the positive pulse current of thesecond leg.

Furthermore, in the second embodiment, the controller 30 changes thephases of the duty ratio instructions so that the time period when totalripple current of a certain phase is minimized and the time period whentotal ripple current of another phase is minimized does not overlap eachother. With such a configuration, superimposition of currents in thenegative or positive direction is reduced concerning the legsconstituting the respective phases, thus reducing ripple current.

Hereinabove, the motor control system according to the embodiments ofthe present invention is described. However, it is obvious that thepresent invention is not limited to the aforementioned embodiments andcan be variously modified without departing from the spirit of theinvention. For example, the above embodiments describe about the motorcontrol system outputting the output power of the power converter to themotor. This is shown by way of example, and the power conversion systemwhich converts inputted power and outputs the same also functions as apart of the present invention. Moreover, the power conversion system canbe applied, in addition to the inverter which receives DC current andoutputs AC power, to a power converter such as a DC/DC converter.

This application is based upon and claims the benefit of priority toJapanese Patent Application No. 2010-158419, filed on Jul. 13, 2010, theentire contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

According to a power conversion system, the phases of the calculatedduty ratio instructions are changed so that the time period whenpositive current flows through a first leg and the time period whennegative pulse current flows through a second leg overlap each other inone control period. This can prevent the superimposition of currents ina same direction. It is therefore possible to reduce ripple current.Accordingly, the controller of a power inverter according to the presentinvention is industrially applicable.

REFERENCE SIGNS LIST

-   -   5 POWER SUPPLY    -   10 MOTOR    -   11 ROTOR    -   12 STATOR    -   20 INVERTER    -   30 CONTROLLER    -   31 TORQUE CONTROL UNIT    -   32 CURRENT CONTROL UNIT    -   33 dq/THREE-PHASE CONVERSION UNIT    -   34 MODULATION FACTOR INSTRUCTION GENERATION UNIT    -   35 PWM CONTROL UNIT    -   36 TIMING CONTROL UNIT    -   37 THREE-PHASE/dq CONVERSION UNIT    -   38 ROTATIONAL SPEED CALCULATION UNIT    -   40 POSITION SENSOR    -   41 CURRENT SENSOR

1-12. (canceled)
 13. A power conversion system converting inputtedelectric power into outputs in a plurality of phases, the systemcomprising: a power converter including a plurality of legscorresponding to each of the phases, each leg having upper and lowerarms; and a controller individually controlling the upper and lower armsof each leg to control pulse current flowing through the leg, whereinthe controller includes: a calculation unit calculating a duty ratioinstruction for each leg in the one control period for each phase; and aphase adjustment unit changing a phase of the duty ratio instructioncalculated by the calculation unit, and the phase adjustment unitchanges the phases of the duty ratio instructions to allow the timeperiod when positive pulse current flows through a first leg and thetime period when negative pulse current flows through a second leg tooverlap each other in the one control period, and the first leg and thesecond leg are provided for a certain one of the phases.
 14. The powerconversion system according to claim 13, wherein the calculation unitsets the duty ratio instructions of the first and second legs equal toeach other.
 15. The power conversion system according to claim 13,wherein the phase adjustment unit compares the time period when positivepulse current flows through the first leg with the time period whennegative pulse current flows through the second leg and changes thephases of the duty ratio instructions to cause the shorter one of thecompared time periods to fall within the other time period.
 16. Thepower conversion system according to claim 13, the system furthercomprising: a smoothing capacitor connected to each phase and each legof the phase, wherein the phase adjustment unit changes the phases ofthe duty ratio instructions to prevent a time period when total ripplecurrent of a certain one of the plurality of phases is minimized and atime period when total ripple current of another phase is minimized fromoverlapping each other.
 17. The power conversion system according toclaim 13, wherein the phase adjustment unit changes the phases of theduty ratio instructions to start the time period when positive pulsecurrent flows through the second leg at the same time as the time periodwhen positive pulse current flows through the first leg ends.
 18. Thepower conversion system according to claim 13, further comprising amotor driven by polyphase power outputted from the power converter,wherein the motor is a electro-mechanical motor which includes the motorand the power converter integrated with each other, and theelectro-mechanical motor includes a plurality of windings and aplurality of bridge circuits composed of the plurality of legs, andoutput points of the legs are individually connected to thecorresponding windings.
 19. The power conversion system according toclaim 18, wherein the number of the legs included in the power converteris an integral multiple of the number of motor slots.
 20. The powerconversion system according to claim 13, wherein among all the legsincluded in the power converter, the number of legs in which any one ofthe upper and lower arms is on remains constant throughout the onecontrol period.
 21. The power conversion system according to claim 13,wherein the phase adjustment unit changes the phases of the duty ratioinstructions to cause the time when on state of the lower arm turns intoon state of the upper arm in the first leg to correspond to the timewhen on state of the upper arm turns into on state of the lower arm inthe second leg and cause the time when on state of the upper arm turnsinto on state of the lower arm in the first leg to correspond to thetime when on state of the lower arm turns into on state of the upper armin the second leg.
 22. A method of controlling a power converter whichis provided with a plurality of legs for each phase to convert electricpower inputted to the power converter into outputs in a plurality ofphases, each leg including upper and lower arms, the method comprising:calculating a duty ratio instruction for each of the legs in one controlperiod for each phase, and changing phases of the calculated duty ratioinstructions so that, concerning first and second legs among theplurality of legs provided for a certain one of the phases, a timeperiod when positive pulse current flows through the first leg and atime period when negative pulse flows through the second leg overlapeach other in the one control period.
 23. A power conversion system,comprising: power conversion means for converting inputted electricpower into outputs in a plurality of phases by using a plurality oflegs, each leg including upper and lower arms; and controlling means forcontrolling the upper and lower arms of each leg to control pulsecurrent flowing through the leg, wherein concerning first and secondlegs among the plurality of legs provided for a certain one of thephases, a time period when positive pulse current flows through thefirst leg and a time period when negative pulse flows through the secondleg overlap each other in the one control period.